Request for Comments: 2950
Category: Standards Track
Telnet Encryption: CAST-128 64 bit Cipher Feedback
Status of this Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright © The Internet Society (2000). All Rights Reserved.
This document specifies how to use the CAST-128 encryption algorithm in cipher feedback mode with the telnet encryption option. Two key sizes are defined: 40 bit and 128 bit.
1. Command Names and Codes
Encryption Type CAST5_40_CFB64 8 CAST128_CFB64 10 Suboption Commands CFB64_IV 1 CFB64_IV_OK 2 CFB64_IV_BAD 3
2. Command Meanings
IAC SB ENCRYPT IS CAST5_40_CFB64 CFB64_IV <initial vector> IAC SE IAC SB ENCRYPT IS CAST128_CFB64 CFB64_IV <initial vector> IAC SE
The sender of this command generates a random 8 byte initial vector, and sends it to the other side of the connection using the CFB64_IV command. The initial vector is sent in clear text. Only the side of the connection that is WILL ENCRYPT may send the CFB64_IV command.
IAC SB ENCRYPT REPLY CAST5_40_CFB64 CFB64_IV_OK IAC SE
IAC SB ENCRYPT REPLY CAST128_CFB64 CFB64_IV_OK IAC SE
IAC SB ENCRYPT REPLY CAST5_40_CFB64 CFB64_IV_BAD IAC SE
IAC SB ENCRYPT REPLY CAST128_CFB64 CFB64_IV_BAD IAC SE
The sender of these commands either accepts or rejects the initial vector received in a CFB64_IV command. Only the side of the connection that is DO ENCRYPT may send the CFB64_IV_OK and CFB64_IV_BAD commands. The CFB64_IV_OK command MUST be sent for backwards compatibility with existing implementations; there really isn't any reason why a sender would need to send the CFB64_IV_BAD command except in the case of a protocol violation where the IV sent was not of the correct length (i.e., 8 bytes).
3. Implementation Rules
Once a CFB64_IV_OK command has been received, the WILL ENCRYPT side of the connection should do keyid negotiation using the ENC_KEYID command. Once the keyid negotiation has successfully identified a common keyid, then START and END commands may be sent by the side of the connection that is WILL ENCRYPT. Data will be encrypted using the CAST128 64 bit Cipher Feedback algorithm.
If encryption (decryption) is turned off and back on again, and the same keyid is used when re-starting the encryption (decryption), the intervening clear text must not change the state of the encryption (decryption) machine.
If a START command is sent (received) with a different keyid, the encryption (decryption) machine must be re-initialized immediately following the end of the START command with the new key and the initial vector sent (received) in the last CFB64_IV command.
If a new CFB64_IV command is sent (received), and encryption (decryption) is enabled, the encryption (decryption) machine must be re-initialized immediately following the end of the CFB64_IV command with the new initial vector, and the keyid sent (received) in the last START command.
If encryption (decryption) is not enabled when a CFB64_IV command is sent (received), the encryption (decryption) machine must be re- initialized after the next START command, with the keyid sent (received) in that START command, and the initial vector sent (received) in this CFB64_IV command.
CAST 64 bit Cipher Feedback
key --->+------+ +->| CAST |--+ | +------+ | | v INPUT --(---------->(+)+---> DATA | | +--------------+
iV: Initial vector, 64 bits (8 bytes) long.
Dn: the nth chunk of 64 bits (8 bytes) of data to encrypt (decrypt). On: the nth chunk of 64 bits (8 bytes) of encrypted (decrypted) output.
V0 = CAST(iV, key) On = Dn ^ Vn V(n+1) = CAST(On, key)
5. Integration with the AUTHENTICATION telnet option
As noted in the telnet ENCRYPTION option specifications, a keyid value of zero indicates the default encryption key, as might be derived from the telnet AUTHENTICATION option. If the default encryption key negotiated as a result of the telnet AUTHENTICATION option contains less than 16 (5) bytes, then the CAST128_CFB64 (CAST5_40_CFB64) option must not be offered or used as a valid telnet encryption option.
If there are less than 32 (10) bytes of key data, the first 16 (5) bytes of key data are used as keyid 0 in each direction. If there are at least 32 (10) bytes of key data, the first 16 (5) bytes of key data are used to encrypt the data sent by the telnet client to the telnet server; the second 16 (5) bytes of key data are used to encrypt the data sent by the telnet server to the telnet client.
Any extra key data is used as random data to be sent as an initialization vector.
6. Security Considerations
Encryption using Cipher Feedback does not ensure data integrity; the active attacker has a limited ability to modify text, if he can predict the clear-text that was being transmitted. The limitations faced by the attacker (that only 8 bytes can be modified at a time, and the following 8-byte block of data will be corrupted, thus making detection likely) are significant, but it is possible that an active attacker still might be able to exploit this weakness.
The tradeoff here is that adding a message authentication code (MAC) will significantly increase the number of bytes needed to send a single character in the telnet protocol, which will impact performance on slow (i.e. dialup) links.
Encryption modes using 40-bit keys are not to be considered secure. The 40 bit key mode CAST5_40_CFB64 is listed here simply to document the implementations that are already prevalent on the Internet but have never been documented.
This document was based on the "Telnet Encryption: DES 64 bit Cipher Feedback" document originally written by Dave Borman of Cray Research with the assistance of the IETF Telnet Working Group.
 Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144, May 1997.
Jeffrey Altman, Editor
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